Most prior art light emitting systems using light emitting diodes (LEDs) would include a light detecting circuit. One of the purposes of using the light detecting circuit is to stabilize the luminous intensity of the light emitting system. The light detecting circuit is used for detecting the luminous intensity of the light emitting system, and the detected luminous intensity is then compared with a preset luminous intensity, and the driving voltage of the LEDs is adjusted based on comparison results, so that the luminous intensity of the light emitting system can be maintained at the preset luminous intensity. The driving voltage is generated by a voltage converter module. In order for the voltage convert module to convert an input voltage into a specified output voltage, a feedback circuit is needed within the voltage converter module. Another purpose of using the light detecting circuit is to save power consumption of the light emitting system. The light detecting circuit is used to detect ambient light. When the detected luminous intensity is higher than a threshold value, it means that the ambient light of the light emitting system is bright enough without activating the light emitting system, so that the LEDs are stopped emitting light until the detected luminous intensity is lower than the threshold value. By then, the LEDs are driven to emit light again.
FIG. 1 illustrates a block diagram of a light emitting system in prior art. As shown, the light emitting system 1 comprises an LED 11, a light detecting circuit 12, a control module 13, an output voltage adjuster 132 and a voltage converter module 14. The voltage converter module 14 is capable of receiving an operating voltage and generating an output voltage 15 to the LED 11 for driving the LED 11 to emit light. The light detecting circuit 12 is capable of detecting luminous intensity of light emitted from the LED 11, and transmits the detected luminous intensity value 121 to the control module 13. The output voltage adjuster 132 is connected between the control module 13 and the voltage converter module 14 and capable of adjusting the voltage of the out signal of the control module to fit the requirement of the voltage converter module 14. The control module 13 compares the detected luminous intensity value 121 with a preset luminous intensity value 131, and then controls the voltage converter module 14 to output the output voltage 15 according to the comparison results. For example, when the detected luminous intensity value 121 is lower than the preset luminous intensity value 131, the control module 13 would control the voltage converter module 14 to raise the output voltage 15, so as to drive the LED 11 to emit even brighter light. And, when the detected luminous intensity value 121 is higher than the preset luminous intensity value 131, the control module 13 controls the voltage converter module 14 to lower the output voltage 15, so that the LED 11 emits less bright light.
The conventional voltage converter module 14 comprises at least a comparison circuit 141, a feedback circuit 142, and a conversion circuit 143. The conversion circuit 143 is controlled by the comparison circuit 141 to raise or lower the output voltage 15. The feedback circuit 142 is connected to the output of conversion circuit 143 to receive the output voltage 15 and feed a feedback signal 144 to the comparison circuit 141. The comparison circuit 141 checks the voltage value fed back by the feedback circuit 142 and then adjusts the conversion circuit 143 according to the checking result.
In the above-described light emitting system 1 in prior art, the light detecting circuit 12 feeds back the luminous intensity value and then the voltage converter module 14 is then controlled to change the output voltage 15. Meanwhile, the voltage converter module 14 also internally comprises a feedback circuit 142 to control the output voltage 15. Therefore, the circuitry design for the light emitting system 1 in prior art has the disadvantages of having low power efficiency and consuming unnecessary power.